This invention relates to a coating applied to a polystyrene pattern for casting metal by a lost foam process. More particularly, this invention relates to a pattern coating applied in a single application and initially having a low permeability to pattern decomposition vapors, but thereafter developing a high permeability, which variable permeability reduces turbulence in metal replacing the pattern, while ultimately venting the vapors, to reduce gas entrapment within the metal and thereby reduce porosity in the product casting.
In copending U.S. patent application Ser. No. 402,108 filed 7/26/82 now U.S. Pat. No. 4,448,235, a lost foam metal casting process is described wherein the polystyrene pattern is coated first with a thermally insulative, relatively gas permeable, refractory layer and thereafter with a vaporizable, relatively gas impermeable, polymeric layer. The refractory layer is applied as a water-base slurry and the polymeric layer as a water-base emulsion. During casting, the refractory layer insulates the polymeric layer to delay vaporization. Because of the low permeability of the polymeric layer, pattern decomposition vapors build up and slow the flow of metal replacing the pattern, thereby reducing vapor-entrapping turbulence in the metal. The polymeric layer eventually vaporizes, leaving the permeable refractory layer, wherethrough pattern decomposition vapors readily vent, thereby avoiding entrapment within the advancing metal. Thus, the two-layer coating produces a desired sequence of low and high permeability that reduces metal turbulence, while ultimately venting vapors, both of which reduce porosity in the cast metal. Although the dual coating has been generally satisfactory for reducing porosity, the two separate coating operations substantially double the required equipment, time and costs. Also, because both coats are water-base, the second coat must be carefully applied to minimize disruption of the first. Although it is desired to eliminate a second coating operation, mixtures of refractory slurry and polymer emulsion have been generally unacceptable, in part because flocculation prevents a suitable coating from being applied.
Therefore, it is an object of this invention to provide a single coating for a vaporizable pattern for lost foam casting of metal, which coating is adapted to sequentially display a relatively low vapor permeability and thereafter a relatively high vapor permeability to cause pattern decomposition vapors to initially build up, but thereafter to vent through the coating, thereby reducing turbulence in the metal, while ultimately venting the vapors, to reduce vapor entrapment in the metal replacing the pattern and thereby to reduce porosity in the casting.
It is a further object of this invention to provide an improved variable permeability, thermally insulative coating suitable for a vaporizable lost foam casting pattern, which coating is adapted to be applied in a single operation to reduce the time, equipment, handling problems and costs associated with multicoat, variable permeability coatings. The coating comprises vaporizable polymeric particles dispersed within a thermally insulative, refractory matrix and is adapted to initially cause pattern decomposition vapors to build up and slow pattern replacement by metal to reduce gas-entrapping turbulence. Thereafter, the polymeric particles vaporize, creating pores in the coating wherethrough vapors escape to avoid entrapment in the metal. Thus, the single coating reduces vapor entrapment to reduce porosity in the cast metal.